1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display device including a switch element, an auxiliary capacitor, and a pixel electrode for each pixel.
2. Description of the Related Art
In recent years, active-matrix liquid crystal display devices have been developed actively. A liquid crystal display device of this type includes a switch element, an auxiliary capacitor, and a pixel electrode for each of segments bordered by a plurality of signal lines and a plurality of scan lines.
As the switch element, for example, a MOS thin-film transistor (TFT) is used. A gate terminal of the TFT is connected to the scan line, a source terminal thereof is connected to the signal line, and a drain terminal thereof is connected to one terminal of the auxiliary capacitor and the pixel electrode. The other terminal of the auxiliary capacitor is connected to a power supply line.
The switch element, the auxiliary capacitor, and the pixel electrode are usually formed on a translucent array substrate. A counter substrate is placed to face the array substrate with a liquid crystal layer interposed therebetween. The pixel electrode on the array substrate and a counter electrode on the counter substrate are placed to face each other with the liquid crystal layer interposed therebetween.
When a scan signal comes in through the scan line, the switch element is turned on, and a video signal voltage which has come in through the signal line is applied through the switch element to the auxiliary capacitor and the pixel electrode. At this time, the potential of the power supply line connected to the auxiliary capacitor is varied. This variation redistributes the charge of the auxiliary capacitor, and determines the voltage to be applied to the pixel electrode. A system in which the voltage on the pixel electrode is determined in this way is called a capacitively coupled drive system. As a liquid crystal display device of this type, for example, one described in Japanese Unexamined Patent Publication No. 2001-255851 is known.
Liquid crystal display devices have wide-ranging uses. In particular, in mobile terminals, there are strong needs for higher definition and higher luminance. In order to sharply display an image such as a photograph, it is required that there be no variations in the gradation-luminance characteristic of a liquid crystal panel.
However, the capacitively coupled drive system has the problem that a gradation shift is prone to occur due to variations in the thickness of a film used to form the auxiliary capacitor.